Synopsis:

Arguments:

The number of descriptors to check in the given sets.
Only the descriptors from 0 through (width-1) in the descriptor
sets are examined.
Therefore, the value of width must be at least as large as:

(highest valued file descriptor in the sets) +1

readfds

NULL, or a pointer to a fd_set object that
specifies the descriptors to check for files that are ready for reading.
The function replaces the set with the file descriptors that are actually
ready for reading.

writefds

NULL, or a pointer to a fd_set object that
specifies the descriptors to check for files that are ready for writing.
The function replaces the set with the file descriptors that are actually
ready for writing.

exceptionfds

NULL, or a pointer to a fd_set object that
specifies the descriptors to check for files that have an exceptional
condition pending.
The function replaces the set with the file descriptors that actually
have an exceptional condition pending.

timeout

NULL, or a pointer to a timeval that
specifies how long to wait for the selection to complete.